Diversity of B and T cell antigen receptor repertoires underlies the ability of adaptive immunity to respond with high specificity to pathogen challenge. The main source of diversity is generated during lymphocyte development via the process of VDJ recombination. As a consequence, each B or T cell expresses one out of many variable (V) gene segments present in the germline. The goals of this project are to understand how B lymphocytes efficiently utilize the entire repertoire of close to one hundred VH gene segments of the immunoglobulin heavy chain (IgH) gene locus. During FY19 we found that: VH gene segments are excluded from a topological chromatin domain within which the VDJ recombinase machinery scans the genome for complementary recombination signal sequences (RSSs) with high efficiency. We used mutated IgH alleles in which the scanning domain is increased in size to examine rules that govern VH gene recombination. On such alleles VH recombination is restricted to 2 out 100 gene segments, and these gene segments recombine without directional preference to either 5 or 3 RSSs of unrearranged DH gene segments. Our interpretation is that VH gene segments find complementary RSSs by a diffusion rather than a scanning mechanism. The altered chromatin loop generated on such alleles also greatly increases recombination efficiency of the selected VH gene segments resulting in reduced diversity. We propose that the chromatin configuration of wild type IgH alleles sacrifices efficiency for diversity. Reduced efficiency may also impose allelic exclusion by increasing recombinational asynchrony between the two alleles in each cell.